(1) Field of Invention
This invention relates to the synthesis of semi-crystalline, mesostructure aluminum oxide materials possessing framework mesopores. In particular, the present invention relates to such materials where the formation of the mesoporous structure is accomplished by a novel molecular assembly mechanism involving various nonionic surfactants, particularly polyethylene oxide based surfactants, and various alumina precursors in the absence of an aluminum alkoxide, which is conventionally used. This nonionic surfactant templating approach allows for the removal of surfactant through calcination or, more preferably, through solvent extraction from the hydrolysis product which lowers material and energy costs. The surfactant is preferably biodegradable. The assembly approach affords non-lamellar mesostructures of aluminum oxide.
(2) Description of Prior Art
Modern human activities rely greatly upon porous solids of both natural and synthetic design. The pore structures of such solids are generally formed during crystallization or during subsequent treatments. These solid materials are classified depending upon their predominant pore sizes: (i) microporous, with pore sizes &lt;1.0 nm; (ii) macroporous, with pore sizes exceeding 50.0 nm; and mesoporous, with pore sizes intermediate between 1.0 and 50.0 nm. Macroporous solids find limited use as adsorbents or catalysts owing to their low surface areas and large non-uniform pores. Micro- and mesoporous solids however, are widely utilized in adsorption, separation technologies and catalysis, particularly in the processing and refining of petroleum. There is an ever increasing demand for new, highly stable well defined mesoporous materials because of the need for ever higher accessible surface areas and pore volumes in order that various chemical processes may be made more efficient or indeed, accomplished at all.
Porous materials may be structurally amorphous, para-crystalline or crystalline. Amorphous materials, such as silica gel or alumina gel, do not possess any crystallographic order, whereas para-crystalline solids such as the transition aluminas .gamma.- or .eta.-alumina are semi-ordered, producing broad X-ray diffraction peaks. Both these classes of materials exhibit very broad pore distributions. This wide pore distribution however, limits the effectiveness of catalysts, adsorbents and ion-exchange systems prepared from such materials. The very broad pore distribution is particularly limiting in the use of these aluminas in petroleum refining.
Those skilled in the art of petroleum hydrobreaking and petroleum hydrocracking will know that the reactivity and selectivity of alumina catalysts used in these processes depends on the pore size distribution and the overall surface area. Narrow pore size distribution in the mesopore range 5-12 nm are especially desired for this purpose.
Zeolites and some related molecular sieves such as; alumino-phosphates and pillared interlayered clays, possess rigorously uniform pore sizes. Zeolites are highly crystalline microporous aluminosilicates where the lattice of the material is composed of IO.sub.4 tetrahedra (I=Al, Si) linked by sharing the apical oxygen atoms. Cavities and connecting channels of uniform size form the pore structures which are confined within the specially oriented IO.sub.4 tetrahedra (Breck, D. W., Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley and Sons; London, pages 1 to 100 (1974)). Zeolites are considered as a subclass of molecular sieves owing to their ability to discriminate small molecules and perform chemistry upon them. Molecular sieves in general are materials with crystalline frameworks in which tetrahedral Si and/or Al atoms are entirely or in part substituted by other atoms such as B, Ga, Ge, Ti, Zr, V, Fe or P. Negative charge is created in the zeolite framework by the isomorphous substitution of Si.sup.4+ ions by Al.sup.3+ or similar ions. In natural zeolites, this charge is balanced by the incorporation of exchangeable alkali or alkaline earth cations such as Na.sup.+, K.sup.+, Ca.sup.2+. Synthetic zeolites utilize these and other cations such as quaternary ammonium cations and protons as charge balancing ions. Zeolites and molecular sieves are generally prepared from aluminosilicate or phosphate gels under hydrothermal reaction conditions. Their crystallization, according to the hereafter discussed prior art, is accomplished through prolonged reaction in an autoclave for 1-50 days and oftentimes, in the presence of structure directing agents (templates). The correct selection of template is of paramount importance to the preparation of a desired framework and pore network. A wide variety of organic molecules or assemblies of organic molecules with one or more functional groups are known in the prior art to provide more than 85 different molecular sieve framework structures. (Meier et al., Atlas of Zeolite Structure types, Butterworth, London, pages 451 to 469 (1992)).
Recent reviews on the use of templates to direct the synthesis of zeolites and molecular sieves, as well as the mechanisms of structure direction, have been produced by Barrer et al., Zeolites, Vol. 1, 130-140, (1981); Lok et al. , Zeolites, Vol. 3, 282-291, (1983); Davis et al., Chem Hater., Vol. 4, 756-768, (1992) and Gies et al., Zeolites, Vol 12, 42-49, (1992). For example, U.S. Pat. No. 3,702,886 teaches that an aluminosilicate gel (with high Si/Al ratio) crystallized in the presence of quaternary tetrapropyl ammonium hydroxide template to produce zeolite ZSM-5. Other publications teach the use of different organic templating agents and include; U.S. Pat. No. 3,709,979, wherein quaternary cations such as tetrabutyl ammonium or tetrabutyl phosphonium ions crystallize ZSM-11 and U.S. Pat. No. 4,391,785 demonstrates the preparation of ZSM-12 in the presence of tetraethyl ammonium cations. Other prior art teaches that primary amines such as propylamine and i-propylamine (U.S. Pat. No. 4,151,189), and diamines such as diaminopentane, diaminohexane and diaminododecane (U.S. Pat. No. 4,108,881) also direct the synthesis of ZSM-5 type structure. Hearmon et al (Zeolites, Vol. 10, 608-611, (1990)) however, point out that the protonated form of the template molecule is most likely responsible for the framework assembly.
In summary, most of the zeolites and molecular sieve frameworks taught in the prior art are assembled by using quaternary ammonium cations or protonated forms of amines and diamines as templates.
The need for new and useful types of stable frameworks and the need to expand the uniform pore size into the mesopore region allowing the adsorption and discrimination of much larger molecules, has driven the search for organic structure-directing agents that will produce these new structures. In the prior art however, molecular sieves possess uniform pore sizes in the microporous range. These pore sizes and therefore the molecular sieving abilities of the materials are predetermined by the thermodynamically favored formation of framework windows containing 8, 10 and 12 I-atom rings. The largest pore size zeolites previously available were the naturally occurring faujasite (pore size 0.74 nm) or synthetic faujasite analogs, zeolites X and Y with 0.8 nm pore windows (Breck, D. W., Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley and Sons; London, pages 1 to 100 (1974)). The innovative use of aluminophosphate gels has allowed the synthesis of new large pore materials. Thus, an 18 I-atom ring aluminophosphate molecular sieve; VPI-5 (Davis et al., Nature, Vol. 331, 698-699, (1988)) was produced and found to consist of an hexagonal arrangement of one dimensional channels (pores) of diameter .apprxeq.1.2 nm. A gallophosphate molecular sieve cloverite, with pore size of 1.3 nm was reported by Estermann M. et al (Nature, Vol 352, 320-323, (1991)), while recently, Thomas J. M. et al (J. Chem. Soc. Chem. Commun., 875-876, (1992)) reported a triethyl ammonium cation directed synthesis of a novel 20 I-atom ring aluminophosphate molecular sieve (JDF-20), with uniform pore size of 1.45 nm (calculated from lattice parameters). A vanadium phosphate material was very recently reported with 1.84 nm lattice cavity (Soghmonian et al., Agwen. Chem. Int. Ed. Engl., Vol. 32, 610-611, (1993)). However, the true pore sizes of the latter two materials are unknown since sorption data were not made available and furthermore, these materials are not thermally stable.
In summary, in spite of significant progress made toward the preparation of large pore size materials, thermally stable molecular sieves were still only available with uniform pore sizes in the microporous range until 1992.
In 1992, a breakthrough in the preparation of mesoporous silica and aluminosilicate molecular sieves was disclosed in U.S. Pat. Nos. 5,098,684 and 5,102,643. The class of mesoporous materials (denoted as M41S) claimed in this prior art was found to possess uniform and adjustable pore size in the range 1.3-10.0 nm. These materials exhibited framework wall thickness from 0.8 to 1.2 nm and elementary particle size generally greater than 50.0 nm. By varying the synthesis conditions, M41S materials with hexagonal (MCM-41), cubic (MCM-48) or layered morphologies have been disclosed (Beck et al., J. Am. Chem. Soc., Vol. 114, 10834-10843, (1992)). The mechanism proposed for the formation of these materials involves strong electrostatic interactions and ion pairing between long chain quaternary alkyl ammonium cations, as structure directing agents, and anionic silicate oligomer species (U.S. Pat. No. 5,098,684). Recently, Stucky et al (Nature, Vol. 368, 317-321 (1994)) extended this assembly approach by proposing four complementary synthesis pathways. The direct co-condensation of anionic inorganic species (I.sup.-) with a cationic surfactant (S.sup.+) to give assembled ion pairs (S.sup.+ I.sup.-), for example MCM-41, was described as Pathway 1. The charge reversed situation with an anionic template (S.sup.-) being used to direct the assembly of cationic inorganic species (I.sup.+) to ion pairs (S.sup.-, I.sup.+) was Pathway 2. Hexagonal iron and lead oxide and lamellar lead and aluminum oxide phases have been reported using Pathway 2 (Stucky et al. ibid.). Pathways 3 and 4 involve the mediation of assemblies of surfactants and inorganic species of similar charge by oppositely charged counterions (X.sup.- .dbd.Cl.sup.-, Br.sup.-, or M.sup.+ =Na.sup.+, K.sup.+). The viability of Pathway 3 was demonstrated by the synthesis of hexagonal MCM-41 using a quaternary alkyl ammonium cation template under strongly acidic conditions (5-10 mol L.sup.-1 HCl or HBr) in order to generate and assemble positively charged framework precursors (Stucky et al. ibid). Pathway 4 was demonstrated by the condensation of anionic aluminate species with an anionic template (C.sub.12 H.sub.25 PO.sub.3.sup.-) via alkali cation mediated (Na.sup.+, K.sup.+) ion pairing, to produce a lamellar Al(OH).sub.3 phase.
All of the aforementioned synthetic pathways involve charge matching between ionic organic directing agents and ionic inorganic precursors. The template therefore, is strongly bound to the charged framework and difficult to recover. For example, in the original Mobil patent (U.S. Pat. No. 5,098,684) the template was not recovered, but burned off by calcination at elevated temperature. Template removal of cationic surfactant has however, been demonstrated by ion-exchange with low pH acidic cation donor solutions (U.S. Pat. No. 5,143,879). Template-halide pairs in the framework of acidic Pathway 3 materials can be partially displaced by ethanol extraction (Stucky et al. Ibid). Thus, ionic template recovery is only possible, if exchange ions or ion pairs are present during the extraction process.
Most recently, the formation of mesoporous molecular sieves via a new route (Pathway 5) was proposed by Pinnavaia et al. (Science, Vol. 267, 865-867, (1995)). In this method, the self assembly of micelles of neutral primary amines (S.degree.) and neutral inorganic alkoxide precursors (I.degree.) was based upon hydrogen bonding between the two components. The new approach (S.degree., I.degree.) taught in that prior art afforded mesostructures with greater wall thicknesses, smaller particle sizes and complementary framework-confined mesoporosities relative to Pathway 1 and 3 materials. In addition, owing to the weak template-framework interactions, Pathway 5 allowed for the facile solvent extraction of the template, removing the need for cation donors or ion pairs. These mesoporous structures are described in U.S. patent application Ser. No. 08/431,310, filed Apr. 28, 1995 and related U.S. patent applications.
Davis and his co-workers have prepared porous aluminas (.about.20 .ANG. pore diameters) by the hydrolysis of aluminum alkoxides in the presence of a carboxylate surfactant as the structure director (Davis et al., Chem. Mater., 8:1451 (1996)). The assembly pathway involved S--I complexation reaction between the surfactant (S) and the inorganic reagent (I), as judged by the presence of IR bands characteristic of chelating carboxylate groups. Yada et al. reported the preparation of hexagonal alumina mesostructures by electrostatic s.sup.- I.sup.+ assembly of dodecylsulfate surfactants and aluminum nitrate (Yada et al., J. Chem. Soc., Chem. Commun., P769, (1996)). However, the mesostructures were not stable to surfactant removal. In contrast, Pinnavaia and Bagshaw have obtained mesoporous alumina molecular sieves, denoted MSU-X, by using nonionic polyethylene oxide surfactants and an aluminum alkoxide as the inorganic precursor (Pinnavaia et al., Science, 269:1242 (1995)), Bagshaw et al, Agwen. Chem. Int. Ed. Engl., 35:1102 (1996) and U.S. Pat. No. 5,622,684). These materials exhibited characteristic wormhole channel motifs. The problem is that aluminum alkoxide reagents are expensive.
In summary, according to the prior art, the molecular sieve materials and preparation techniques provide several distinct disadvantages and advantages:
i) The prior art of Pathways 1 through 4 teaches the use of charged surfactant species as templates in order to assemble inorganic frameworks from charged inorganic precursors. These charged templates are generally expensive, strongly bound to the inorganic framework and therefore difficult to recover. Additionally, many of these templates such as the most commonly used quaternary ammonium cations are highly toxic and environmentally undesirable. In the prior art of Pathways 1 to 4, the template was removed from the structure by either calcining it out or by ion-exchange reactions. Pathway 5 prior art templates are also highly toxic and environmentally unsuitable, but may be removed through environmentally benign ethanol extraction and thereby recovered and reused. No thermal stable mesostructured alumina has been produced through pathways 1 to 5.
There is a need for new methods of preparation of new materials of these types, cost reductions, ease of recoverability and environmental compatibility in the template and inorganic precursors has lead to the development of a new synthetic method to be described herein.